maplist([X]>>(test(X,Xs)) 可以在范围外看到变量吗? [英] Can maplist([X]>>(test(X,Xs)) see variable in outer in scope?

问题描述

?- maplist([X]>>(member(X,[1,2])),
        [1,2]).
true

但是:

?- X2s=[1,2],
   X1s=[1,2],
   maplist([X]>>(member(X,X1s)),
           X2s).
X1s = X2s, X2s = [1, 2]

X1s 未接地，尽管已接地.

X1s is not grounded, despite being grounded.

这是一个测试它的笔记本:https://swish.swi-prolog.org/p/ungrounded%20in%20map.swinb

Here a notebook testing it: https://swish.swi-prolog.org/p/ungrounded%20in%20map.swinb

这是怎么回事?

推荐答案

不要被 SWI-Prolog 变量名别名混淆

Do not get confused by the SWI-Prolog variable-name-aliasing

?- debug(foo),X2s=[1,2],
   X1s=[1,2],
   maplist([X]>>(debug(foo,"Testing ~q",[X]),member(X,X1s)),X2s).

Warning: foo: no matching debug topic (yet)
% Testing 1
% Testing 2
X2s = X1s, X1s = [1, 2] ;
false.

只是意味着

• 查询成功，X 按预期打印(因此 X1s 和 X2s 在 maplist/3 中可见>.
• X2s 和 X1s 是同一个词条的别名(这是对的)，[1, 2](也是对的).
• 重试失败.

• The query succeeds with the X printed as expected (so X1s and X2s are visible insidemaplist/3.
• X2s and X1s are aliases for the same term (which is true), [1, 2] (also true).
• A retry fails.

实际上，maplist/3 不会屏蔽"子句上下文中可见的任何变量，但是 yall lambda 符号 有一种方法可以指定 lambda 表达式 >> 是指外部上下文中的变量，{}:

Actually, maplist/3 doesn't "shield" from any variables visible in the clause context, but the yall lambda notation has a way of specifying that the lambda expression >> is referring to variables in the outer context, {}:

?- debug(foo),X2s=[1,2],
   X1s=[1,2],
   maplist({X1s}/[X]>>(debug(foo,"Testing ~q",[X]),member(X,X1s)),X2s).

尽管 {X1s} 在这里没有做任何事情.我不知道它在哪里...... Prolog 中没有正确完成屏蔽变量"(恕我直言，它应该，尤其是在 bagof/3 和 setgof/3>)

Although the {X1s} properly doesn't do anything here. I don't know where it does ... "shielding off variables" is not properly done in Prolog (IMHO, it should, especially in bagof/3 and setgof/3)

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